Hyperparathyroidism in man is a syndrome characterized by elevated parathormone secondary to at least two disorders: 1) Diffuse hyperplasia of all four parathyroid glands and 2) parathormone production by a single adenoma. Histologic distinction between single and multiple gland disease is frequently impossible, yet this is desirable, since treatment is different. Known adenomas are being compared to known hyperplastic tissue biochemically and ultrastructurally; criteria developed from this study group will then be applied to a larger group which is of uncertain etiology in an attempt to separate patients with diffuse hyperplasia from those with adenomas. Unique morphologic features are being sought in an effort to discriminate between the two groups at the time of initial surgery. Ultrastructural analysis of parathyroid tissue is being compared to other criteria namely: 1) Clinicopathological diagnosis; 2) in vitro calcium suppressibility of parathyroid cells; and 3) ABO surface antigen presence or absence.